Back in the days of meatspace, your sense of community largely depended on geography. You couldn't form the kind of close bonds with people far away that you could with those nearby. That created not merely insular communities, but a homogeneity of insularity. Your geographically-based community would evidence insularity: they would have common parlance, inside jokes, groupthink; they would influence each other's thinking. And those people would be the only ones you'd form these communities with. There might be subsets within the larger community -- all the bridge players within the neighborhood, for instance -- but there'd still be a homogeneity of insularity because the boundaries that kept you with these people applied to every part of your life. All your various sub-communities would be drawn from the same group of people.
As time has passed, this homogeniety has broken down bit by bit, because of advances in industrialization, transportation, and most powerfully, communications. Generations of sociologists have analyzed the changes in society caused by this phenomenon. The Internet is merely the latest demesne in which such boundaries get further eroded, and it is by far the most febrile; information flows more freely than ever before.
More specifically, information flows by its own pressure now. Telephones made it possible to form communities not bound by geography, as had automobiles and telegrams and the Roman roads before them to varying extents, but in each case, information flowed only because a person made a conscious effort to direct it to a particular location, typically at some cost in time, effort, or money. Now, any dweeb can post a dumb blog, or a home page, and thanks to spider-bots crawling the Web feeding search engines and link farms, people he has never met and would never meet can and will read his words. He can put up a wiki and strangers can contribute to his ideas. The information now has its own legs and goes places without any active effort on the part of the person who had it, and this breaks down the boundaries that form insular communities more than any earlier change did.
And yet... and yet! For all this, humans are still naked apes. They still feel a hardwired, inescapable need to form communities. They still cluster into insular groups, bond together, exclude outsiders, have inside jokes and groupthink. The difference now is not the absence of insularity; it is the absence of the homogeneity of that insularity.
Put in concrete terms: I am a member of dozens of separate, unrelated Internet communities. There are people I could call community-comrade in one, that I will never encounter in another. I may see some common people in my TiVo, Prius, Home Automation, and Roomba communities, but only a few, and likely no crossover between any of those and my MUD, Roleplaying, or Environmentalism communities, for example.
Each of these communities remains insular. But every member of them is also the member of dozens of other equally-insular communities. We are still social animals, we just learned how to multiplex our social impulses over parallel simultaneous channels. We've become full-duplex.
And yet to a surprising degree, all the sociological rules still mysteriously keep applying to these insular communities. One noteworthy one I have had several run-ins with lately is the cultural paranoia about something that "stung us once", and has become its own Vice and Virtue within the groupthink since.
An insular community is tooling along through its lifespan when it hits a problem; something happens that leads to something bad. Following perfectly natural human tendencies, the community elevates the original source of the badness to a grand evil and become, essentially, paranoid about it. Once bitten, twice shy. Years later, this aversion has become inculcated into the culture; it is now a Law, a Decree: thou shalt not do whatever it was that stung us all umpteen years ago.
Meanwhile, in other similar communities all over the place, that same activity is happening without causing any catastrophes. But in each one, some other thing caused a catastrophe, and so has become anathema to the community by the same mechanism. Each community is paranoid equally, but about different things.
Cross from one community to another, and you might get thumped with culture shock. Something you always did and never had a problem with is reviled as a Great Evil, via a groupthink that is impenetrable; it has long eclipsed any possibility of rationality, as everything is consumed in layers of repetition of the mantra, memories of the painful moment. Meanwhile, something you always thought was and should be forbidden, a source of Bad Things, is being routinely practiced and causing no problems, and people are reacting with surprise to any intimation that it could ever have been a trouble source.
The breakdown of homogeneity of insular communities lets us have this culture shock and see this phenomenon. It does not yet, at least so far as I can see, provide a means of resolving it. No amount of "But they're doing it over there and nothing bad is happening! You just had an unfortunate bit of bad luck with it once!" will actually break down the cultural paranoia within an insular community. All you can do, as with any established insular community, is to adapt and conform. Maybe someday by being on the inside (if you ever break through the barriers and become a true insider) you could influence the groupthink away from it. More likely, by then, you'll have conformed with it, possibly unknowingly.